The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such an application which has improved photodetector means for detecting the Position or level of a reference Plane defined by a rotating laser beam or defined by a stationary plane of laser light.
Laser systems have been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser projector device which provides such a rotating reference beam. The rotating beam define a plane, and various measurements can be made using the as a reference For example, the elevation of a point remote from the laser beam projector device may be measured by means of a rod on which a laser beam detector is mounted. The bottom of the rod rests on the ground at the Point where the measurement is to be made, and the operator moves the detector along the rod to a position where it intercepts the laser beam, as indicated by a meter or other display on the detector device. One such detector device is shown in U.S. Pat. No. 4,240,208, issued June 30, 1987, to Petersen.
A similar surveying system is shown in U.S. Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al. In the Cain et al system, a rotating beam is not used. Rather, a laser transmitter produces an alignment field by projecting laser energy in a non-planar, stationary reference cone. When the transmitter is level, this reference cone declines from the horizontal in an amount sufficient so that some compensation is provided for positional errors which occur due to the curvature of the earth. The Cain et al patent discloses a small, hand-held device which includes a display and a photodetector module.
Both of the detector devices shown in the Cain et al and Petersen patents include a pair of adjacent, triangularly shaped photodetector elements. The orientation of the photodetector elements is such that the changes in the signal outputs from the elements which occur due to movement of the light are inversely related. As the reference light moves upward, the signal output from one of the elements increases while the signal output from the other of the elements decreases. The opposite signal changes occur when the position of the reference light moves downward. Naturally, the signal levels are also affected by the intensity of the light source and the distance of the photodetector elements from the light source. By comparing the relative signal output levels from the two photodetector elements, however, it is possible to obtain an indication of the position of the reference light without regard to the absolute intensity of the light.
While such a detector arrangement provides accurate operation under varying conditions, it has been found to be subject to error in instances when the detector device is improperly oriented. The photodetector elements are positioned behind an aperture in the case of the detector device. If the detector device is misaligned by rotation about a vertical axis such that the light does not strike the elements at normal incidence, shading of the elements by the case at the edge of the aperture can occur. If the angle of incidence is large, the photodetector elements may be entirely shaded and the device will not function. This is an inconvenience for the operator, but is simply corrected by realignment of the device. More serious, however, is the situation in which the photodetector elements are only partially shaded. In this event, the two elements are not shaded equally, thereby producing a shift in the relative signal levels from the elements, and a corresponding error in the detected position of the reference light. Since the device provides an indication of light position, the operator may be unaware of the misalignment of the device and the resulting error in its operation.
One approach for improved detection and display of the relative position of a generally horizontal reference plane of light, even when partial shading occurs, is disclosed in the parent application, allowed U.S. patent application, Ser. No. 228,465, filed Aug. 5, 1988, by Ake now U.S. Pat. No. 4,907,874. The parent application discloses a detection and display device comprising photodetector means having first and second interdigitated photodetector elements positioned adjacent each other on the device, a circuit for determining the relative levels of detection signals produced by the detector elements, and a display. The photodetector elements are each made up of a plurality of sections which are arranged in a generally vertically oriented row. The heights of the sections of one of the elements increase from the bottom of the row to the top of the row, while the heights of the sections of the other element decrease from the bottom of the row to the top of the row. By this arrangement, shading of the elements along a vertical edge of the aperture does not affect the relative areas of the elements illuminated by a beam of light.
It has been found that this detection device responds accurately using a variety of helium-neon transmitters, regardless of partial shading, because the beam typically produced by a helium-neon transmitter is round in cross-section and has a Gaussian energy distribution. However, if the beam is narrow and too few of the sections of the elements are illuminated, a non-uniform response may occur. This occurs most often with laser diode transmitters where the resulting beam tends to be generally elliptically shaped. Where the generally elliptically shaped beam assumes an orientation in which the major axis of the spot of light falling on the cells is vertical, sufficient illumination of enough sections of both the first and second interdigitated photodetector elements occurs. A problem arises, however, when the generally elliptically shaped spot assumes an orientation in which its major axis is horizontal, in which case the vertical dimension of the beam is quite small and the number of illuminated cells is reduced.
An additional problem associated with this configuration of interdigitated photodetector elements disclosed in the parent application is the possibility of a nonlinear output resulting from a beam having a constant energy distribution, even when the beam is wide. As the beam is shifted upward, remaining a constant width, the relative outputs will remain constant until the bottom of the beam shifts out of the area of one element and the top of the beam shifts into the area of the other element. This results in a stair-step cell output ratio, instead of the desired linear response.
Accordingly, it is seen that there is a need for an improved device for detecting and displaying the relative position of reference light wherein the linear response is independent of spot size and energy distribution.